In a two-node bi-directional Free Space Optical (FSO) communication system, the two FSO nodes exchange data encoded on optical carrier beams sent across an unobstructed line of sight (LOS) between the two nodes. As shown in FIG. 1, a conventional two-node bi-directional system is illustrated. As shown, a first node 2 and a second node 3 communication by transmitting and receiving a signal 6, 7 sent between the nodes. The data can be encoded on the signals in any matter; a binary, on-off, exemplary signal is illustrated for simplicity. Each node has an optical output 4 for transmitting the desired signal 6, 7, and also an optical input 5 for receiving the transmitted signal. Once received, the internal electronics of the node can decode the signal and obtain the transmitted data.
FSO links operating through the atmosphere are subject to the effects of scintillation. Scintillation is due to small temperature induced changes in the optical index of refraction across different portions of the optical beam as it propagates from one terminal to another. When illuminating a far end aperture, the result is widely varying received power in the aperture, typically referred to as power in the bucket. In order to provide robust and reliable data transmission with high availability and throughput, techniques are required to overcome this atmospheric induced fading.
Other approaches to provide high reliability FSO links through the atmosphere include transmitting higher optical power, the use of adaptive optics, or the use of multiple receive apertures:
Higher transmit power—By transmitting additional optical power such that even during the deepest fades there is enough received power to close the link, a robust link can be maintained. However, this is highly inefficient since a vast majority of the time the transmitter is transmitting significantly more power than is required. This increases system power consumption, system complexity and system cost.
Adaptive optics—Adaptive Optics (AO) systems attempt to measure the atmospheric induced wavefront distortion and transmit a pre-distorted wavefront such that the received optical signal at the far end terminal has reduced power fluctuations. Such systems require a wavefront sensor to measure the received waveform, a mechanism (typically a deformable mirror) to predistort the optical beam, and a control system to use the wavefront measurements to control the wavefront pre-distortion. Such approaches can provide some benefit in static conditions. However, in dynamic conditions, the limited operating bandwidth of such systems limits their usefulness. In addition, AO systems add considerable complexity to the system, adding cost and size limiting the usefulness for applications that have limited size and power requirements.
Larger apertures or Multiple receive apertures—The last approach to overcome this fading is to use larger apertures or use multiple receive apertures. If portions of the beam can be gathered at distances greater than the spatial correlation distance of the received optical beam, than these approaches will reduce the amount of fading present. However, these approaches increases system size, weight and cost, limiting their usefulness to develop compact FSO systems.